1. Field of the Invention
The present invention relates to a heating device for heating various materials and devices and more particularly to a fixing device for fixing a toner image formed on a sheet-like recording medium by using a heating device and an image forming apparatus including the fixing device.
2. Description of the Background Art
A copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus includes a fixing device for fixing a toner image formed on a paper sheet or similar sheet with heat and pressure. The fixing device usually includes a fixing member in the form of a roller or a belt and a pressing member in the form of a roller, a belt or a pad. The fixing member and pressing member cooperate to fix the toner image on the sheet being passed through a nip therebetween.
At least one of the fixing roller and pressing roller, for example, is implemented as a heat roller to be heated by a heater or heat source. A thermistor or similar temperature sensor is pressed against the heat roller via a polyimide resin film or similar protection film, sensing the surface temperature or the heat roller. For the heater, a halogen heater using a halogen lamp is used. A CPU (Central Processing Unit) controls power source from a commercially available power source to the halogen heater. At this instant, the CPU controls the power source such that the surface temperature of the heat roller remains at a preselected value in accordance with the output of the temperature sensor. A thermostat or similar safety device adjoins the surface of the hat roller and shuts off power source to the halogen heater only when the surface temperature of the heat roller rises above a preselected upper limit.
Today, energy saving is one of important issues even in the image forming art from the environment standpoint. As for the image forming apparatus, the fixing device consumes substantial energy in fixing a toner image on a sheet. It is a common practice to maintain, in a stand-by state, the heat roller at a temperature slightly lower than a fixing temperature for thereby saving energy. When the apparatus is to be used, the temperature of the heat roller is immediately raised to the fixing temperature to thereby prevent the operator from wasting time. However, even in the stand-by state, some power is fed to the fixing device, wastefully consuming energy. It has been reported that the energy consumption of the fixing device in the stand-by state amounts to about 70% to 80% of the total energy consumption of the apparatus.
In light of the above, there is an increasing demand for an implementation that reduces power supply to the fixing unit to practically zero in the stand-by sate. This, however, forces the operator to simply wait for a period of time as long as several minutes to ten and several minutes, which is necessary for the heat roller to be again heated to the fixing temperature, e.g., 180xc2x0 C. or so. This is because the heat roller is usually formed of iron, aluminum or similar metal.
While the surface temperature of the heat roller should immediately be raised to the fixing temperature (within less than 10 seconds) at the time of image formation, power that can be supplied to the heat roller is limited. Further, the heat roller has a great thermal capacity and therefore needs a long warm-up time from the stand-by state. It is therefore necessary to preheat, in the stand-by state, the heat roller for thereby maintaining the surface temperature of the heat roller around a fixable temperature. Preheating consumes much energy despite that the fixing device is not operating. However, if the warm-up time is as short as 5 seconds to 10 seconds, then it is possible to obviate preheating or to preheat the heat roller only to a temperature far lower than the conventional temperature, thereby preventing the operator from wasting time.
To reduce the warm-up time, the tubular base of the heat roller is provided with wall thickness as small as 1 mm to 0.25 mm in order to reduce thermal capacity. The thin wall configuration, however, critically reduces the mechanical strength of the heat roller and causes the roller to easily collapse or deform. Moreover, the thin wall configuration is not attainable without resorting to sophisticated, precision machining technologies, resulting in an increase in cost.
The warm-up time will be reduced if much power can be fed to a heater that heats the heat roller. However, a 100 V, 15A commercial power source is usually shared by the heater, sheet conveying system, image forming section and controller included in the image forming apparatus. While greater power is used for large-scale image forming apparatuses, such apparatuses need extra work for obtaining the greater power with the commercial power source and are limited in location. A chargeable battery is capable of implementing rapid warm-up from the stand-by state without regard to the limit of the commercial power source. A chargeable battery, however, brings about a problem that if a temperature controller is disabled due to some error, then energy continuously fed to the heater at the time of warm-up causes the fixing temperature to sharply rise above an upper limit, resulting in a fire or similar dangerous occurrence.
Moreover, at the beginning and end of the supply of great current, a sharp change in current or a rush current increases a load on the member to be heated. In addition, the rush current flows even to peripheral circuits and produces noise. For this reason, power source from a large capacity, auxiliary power source should not be frequently turned on and turned off. Moreover, instantaneous supply of great power is apt to heat the subject member to an excessive degree.
Safety is another problem with a fixing device featuring an extremely short warm-up time. The temperature of a conventional fixing device of the type continuously receiving constant energy from a commercial power source continuously rises even when temperature control is disabled due to an error. If the temperature elevation is extremely sharp, then a thermostat or safety device cannot follow the temperature elevation and is apt to cause a sheet to ignite.
Various technologies for solving the problems discussed above have been proposed in the past. Japanese Patent Laid-Open Publication No. 10-10913, for example, proposes to feed, in a stand-by state, a voltage lower than a usual voltage by a preselected level to a heat roller to thereby slow down the drop of the temperature of a fixing device. Japanese Patent Laid-Open Publication No. 10-282821 proposes to charge a secondary battery or auxiliary power source in a stand-by state and feed, at the time of warm-up, feed power from both of a main power source and the auxiliary power source, thereby reducing the warm-up time.
Japanese Utility Model Laid-Open Publication No. 63-150967 discloses a fixing device including a first and a second heater respectively powered by an AC power source and a battery that is charged by charging means.
Japanese Patent Laid-Open Publication 3-5779 teaches an image forming apparatus including a fixing device including a press roller that accommodates a main heater and a subheater therein. In this apparatus, a main power source and a storage battery heat the main heater and subheater, respectively. First switching means selectively turns on or turns off the main power source. Charging means charges the storage battery. Second switching means selectively connects the storage battery to the subheater or to the charging means. Temperature sensing means senses the temperature of the press roller. Control means controls the first and second switching means in accordance with the output of the temperature sensing means. When the temperature of the press roller drops below a reference temperature relating to a fixing ability, the control means causes the storage battery to heat the subheater. When the above temperature rises above the reference temperature, the controller stops heating the subheater.
Japanese Patent Laid-Open Publication No. 3-36579 discloses a heating device for fixation including a heater that heats by being supplied with power via heater drive means. The heater drive means includes a chargeable storage battery and a charger connected to a commercial power source for charging the storage battery. The heater is made up of a main heater powered by the commercial power source and an auxiliary heater powered by the storage battery. The storage battery is selectively connected to the charger in the form of a charging circuit or to the auxiliary heater in the form of a discharging circuit. The connection that forms the discharging circuit reduces a warm-up time.
Japanese Patent Laid-Open Publication No. 2000-98799 proposes a heating device for fixation including a heater that heats by being applied with power and heater drive means for feeding power to the heater. The heater drive means includes a chargeable storage battery and a charger connected to a commercial power source for charging the storage battery. The heater includes a main and an auxiliary heater respectively powered by the commercial power source and storage battery. The storage battery is charged when the main heater is turned off.
Other technologies relating to the present invention are disclosed in e.g., Japanese Utility Model No. 7-41023, 10-232821, 2000-315567 and 2001-66926.
It is an object of the present invention to provide a heating device capable of saving power and obviating noise ascribable to a rush current and a sharp change in current when great current is supplied, a fixing device using the same, and an image forming apparatus including the fixing device.
It is another object of the present invention to provide a heating device capable of being rapidly warmed up from a stand-by state without regard to the limit of a commercial power source, a fixing device using the same, and an image forming apparatus including the fixing device.
It is still another object of the present invention to provide a heating device free from excessive temperature elevation, a fixing device using the same, and an image forming apparatus including the fixing device.
It is a further object of the present invention to provide a heating device insuring safety when temperature control is disabled.
A heating device of the present invention includes a main power source and an auxiliary power source implemented by a chargeable capacitor. A heater is made up of a main heating element that heats when supplied with power from the main power source and an auxiliary heating element that heat when supplied with power from the auxiliary power source. A charger charges the capacitor of the auxiliary power source when supplied with power from the main power source. A switch selectively causes the auxiliary power source to be charged or to feed power to the auxiliary heating element. A controller adjusts the power to be fed from the auxiliary power source to the auxiliary heating element.
A fixing device using the heating device of the present invention and an image forming apparatus including the fixing device are also disclosed.